Update minimum CMake version to 2.8.12
Update minimum CMake version to 2.8.12

Latest CMake has deprecated support for CMake
older than 2.8.12:
https://cmake.org/cmake/help/latest/release/3.19.html#deprecated-and-removed-features

salilab/rmf,salilab/rmf,salilab/rmf,salilab/rmf